Synchronization signal transmission state reporting method performed by terminal in wireless communication system and terminal using same

ABSTRACT

Provided are a synchronization signal transmission state reporting method performed by a terminal in a wireless communication system and a terminal using the method. The method comprises: evaluating an event related to the synchronization signal transmission state by the terminal; and when the event is satisfied, transmitting a report for the synchronization signal transmission state.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to wireless communication, and more particularly to a synchronization signal transmission state reporting method performed by a terminal in a wireless communication system and a terminal using the same.

Related Art

In an International Telecommunication Union Radio communication sector (ITU-R), a standardization of International Mobile Telecommunication (IMT)-Advanced being a next mobile communication system after a third generation has been performed. The IMT-Advanced is aimed at supporting an Internet Protocol (IP) based multi-media service with a data transmission rate of 1 Gbps in a stop and low speed moving state and a data transmission rate of 1 Gbps in a high speed moving state.

A 3rd Generation Partnership Project (3GPP) is preparing LTE-Advanced (LIE-A) being an improved one of Long Term Evolution (LTE) based on an OFDMA (Orthogonal Frequency Division Multiple Access)/SC-FDMA (Single Carrier-Frequency Division Multiple Access) transmission scheme as a system standard satisfying requirements of IMT-Advanced. The LTE-A is one important candidate for IMT-Advanced.

In recent years, there is growing interest in a Device-to-Device (D2D) technology performing direct communication between devices. In particular, the D2D is attracting attention as a communication technology for a public safety network. A commercial communication network has been rapidly changed to the LTE but a current public safety network is based on a 2G technology in a collision problem and a cost side with an existing communication standard. Request for the technology clearance and an improved service induces an effort to improve the public safety network.

The public safety network has high service requirements (reliability and security) as compared with a commercial communication network. In particular, when coverage of cellular communication is insufficient or is not used, there is a need for direct signal transmission/reception between devices, that is, an D2D operation.

The D2D operation may be signal transmission/reception between adjacent devices to have various advantages. For example, a D2D terminal may perform data communication with a high transmission rate and low delay. Further, the D2D operation may distribute traffic converged in a base station. If the D2D terminal serves as a relay, the D2D terminal may serve to extend coverage of a base station.

According to the related art, a terminal performs synchronization using a synchronization signal transmitted from a base station to transmit and receive a signal according thereto. Accordingly, there was no reporting with respect to the synchronization signal. Meanwhile, a device-to-device (D2D) operation may be performed between terminals. In this case, for transmission and reception/decoding of a wireless signal, synchronization should be achieved between terminals performing the D2D operation. However, if at least one of the terminals is located outside of coverage of a base station, the terminal cannot receive a synchronization signal, synchronization according to the related art is impossible.

Accordingly, there is a need for a method and an apparatus for synchronization between terminals performing a D2D operation.

SUMMARY OF THE INVENTION

The present invention provides a synchronization signal transmission state reporting method performed by a terminal in a wireless communication system and a terminal using the same.

In an aspect, a synchronization signal transmission state reporting method performed by a user equipment (UE) in a wireless communication system is provided. The method comprises evaluating an event related to the synchronization signal transmission state by the UE and when the event is satisfied, transmitting a report for the synchronization signal transmission state.

In another aspect, a user equipment (UE) is provided. The UE comprises an RF unit configured to send and receive radio signals and a processor connected to the RF unit to be operated, wherein the processor evaluates an event related to the synchronization signal transmission state by the UE and transmits a report for the synchronization signal transmission state when the event is satisfied.

According to the present invention, the terminal may transmit a synchronization signal for a different neighboring terminal. The different terminal receiving the synchronization signal may perform synchronization to perform a D2D operation, may perform synchronization with another terminal receiving the synchronization to perform the D2D operation, or may perform synchronization with a network to communicate with. That is, the D2D operation with respect to various situations is possible so that a reliable D2D operation is possible.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a wireless communication system to which the present invention is applied.

FIG. 2 is a diagram showing a wireless protocol architecture for a user plane.

FIG. 3 is a diagram showing a wireless protocol architecture for a control plane.

FIG. 4 is a flowchart illustrating the operation of UE in the RRC idle state.

FIG. 5 is a flowchart illustrating a procedure of establishing RRC connection.

FIG. 6 is a flowchart illustrating an RRC connection reconfiguration procedure.

FIG. 7 is a diagram illustrating an RRC connection re-establishment procedure.

FIG. 8 illustrates sub states where the terminal may have in an RRC_IDLE state and a sub state transition process.

FIG. 9 illustrates a reference structure for a ProSe.

FIG. 10 illustrates arrangement examples of terminals performing ProSe direct communication and cell coverage.

FIG. 11 illustrates a user plane protocol stack for the ProSe direct communication.

FIG. 12 illustrates a PC 5 interface for D2D discovery.

FIG. 13 illustrates an embodiment of a ProSe direct discovery procedure.

FIG. 14 illustrates another embodiment of a ProSe direct discovery procedure.

FIG. 15 illustrates an example of transmitting a synchronization signal by a terminal for another terminal.

FIG. 16 illustrates another example of transmitting a synchronization signal by the terminal for another terminal.

FIG. 17 illustrates a synchronization signal transmission state reporting method performed by a terminal.

FIG. 18 illustrates a synchronization signal transmission state reporting method according to an embodiment of the present invention.

FIG. 19 illustrates a synchronization signal transmission state reporting method according to another embodiment of the present invention.

FIG. 20 is a block diagram illustrating a wireless device according to an embodiment of the present invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 shows a wireless communication system to which the present invention is applied. The wireless communication system may also be referred to as an evolved-UMTS terrestrial radio access network (E-UTRAN) or a long term evolution (LTE)/LTE-A system.

The E-UTRAN includes at least one base station (BS) 20 which provides a control plane and a user plane to a user equipment (UE) 10. The UE 10 may be fixed or mobile, and may be referred to as another terminology, such as a mobile station (MS), a user terminal (UT), a subscriber station (SS), a mobile terminal (MT), a wireless device, etc. The BS 20 is generally a fixed station that communicates with the UE 10 and may be referred to as another terminology, such as an evolved node-B (eNB), a base transceiver system (BTS), an access point, etc.

The BSs 20 are interconnected by means of an X2 interface. The BSs 20 are also connected by means of an S1 interface to an evolved packet core (EPC) 30, more specifically, to a mobility management entity (MME) through S1-MME and to a serving gateway (S-GW) through S1-U.

The EPC 30 includes an MME, an S-GW, and a packet data network-gateway (P-GW). The MME has access information of the UE or capability information of the UE, and such information is generally used for mobility management of the UE. The S-GW is a gateway having an E-UTRAN as an end point. The P-GW is a gateway having a PDN as an end point.

Layers of a radio interface protocol between the UE and the network can be classified into a first layer (L1), a second layer (L2), and a third layer (L3) based on the lower three layers of the open system interconnection (OSI) model that is well-known in the communication system. Among them, a physical (PHY) layer belonging to the first layer provides an information transfer service by using a physical channel, and a radio resource control (RRC) layer belonging to the third layer serves to control a radio resource between the UE and the network. For this, the RRC layer exchanges an RRC message between the UE and the BS.

FIG. 2 is a diagram showing a wireless protocol architecture for a user plane. FIG. 3 is a diagram showing a wireless protocol architecture for a control plane. The user plane is a protocol stack for user data transmission. The control plane is a protocol stack for control signal transmission.

Referring to FIGS. 2 and 3, a PHY layer provides an upper layer with an information transfer service through a physical channel. The PHY layer is connected to a medium access control (MAC) layer which is an upper layer of the PHY layer through a transport channel. Data is transferred between the MAC layer and the PHY layer through the transport channel. The transport channel is classified according to how and with what characteristics data is transferred through a radio interface.

Data is moved between different PHY layers, that is, the PHY layers of a transmitter and a receiver, through a physical channel. The physical channel may be modulated according to an Orthogonal Frequency Division Multiplexing (OFDM) scheme, and use the time and frequency as radio resources.

The functions of the MAC layer include mapping between a logical channel and a transport channel and multiplexing and demultiplexing to a transport block that is provided through a physical channel on the transport channel of a MAC Service Data Unit (SDU) that belongs to a logical channel. The MAC layer provides service to a Radio Link Control (RLC) layer through the logical channel.

The functions of the RLC layer include the concatenation, segmentation, and reassembly of an RLC SDU. In order to guarantee various types of Quality of Service (QoS) required by a Radio Bearer (RB), the RLC layer provides three types of operation mode: Transparent Mode (TM), Unacknowledged Mode (UM), and Acknowledged Mode (AM). AM RLC provides error correction through an Automatic Repeat Request (ARQ).

The RRC layer is defined only on the control plane. The RRC layer is related to the configuration, reconfiguration, and release of radio bearers, and is responsible for control of logical channels, transport channels, and PHY channels. An RB means a logical route that is provided by the first layer (PHY layer) and the second layers (MAC layer, the RLC layer, and the PDCP layer) in order to transfer data between UE and a network.

The function of a Packet Data Convergence Protocol (PDCP) layer on the user plane includes the transfer of user data and header compression and ciphering. The function of the PDCP layer on the user plane further includes the transfer and encryption/integrity protection of control plane data.

What an RB is configured means a procedure of defining the characteristics of a wireless protocol layer and channels in order to provide specific service and configuring each detailed parameter and operating method. An RB can be divided into two types of a Signaling RB (SRB) and a Data RB (DRB). The SRB is used as a passage through which an RRC message is transmitted on the control plane, and the DRB is used as a passage through which user data is transmitted on the user plane.

If RRC connection is established between the RRC layer of UE and the RRC layer of an E-UTRAN, the UE is in the RRC connected state. If not, the UE is in the RRC idle state.

A downlink transport channel through which data is transmitted from a network to UE includes a broadcast channel (BCH) through which system information is transmitted and a downlink shared channel (SCH) through which user traffic or control messages are transmitted. Traffic or a control message for downlink multicast or broadcast service may be transmitted through the downlink SCH, or may be transmitted through an additional downlink multicast channel (MCH). Meanwhile, an uplink transport channel through which data is transmitted from UE to a network includes a random access channel (RACH) through which an initial control message is transmitted and an uplink shared channel (SCH) through which user traffic or control messages are transmitted.

Logical channels that are placed over the transport channel and that are mapped to the transport channel include a broadcast control channel (BCCH), a paging control channel (PCCH), a common control channel (CCCH), a multicast control channel (MCCH), and a multicast traffic channel (MTCH).

The physical channel includes several OFDM symbols in the time domain and several subcarriers in the frequency domain. One subframe includes a plurality of OFDM symbols in the time domain. An RB is a resources allocation unit, and includes a plurality of OFDM symbols and a plurality of subcarriers. Furthermore, each subframe may use specific subcarriers of specific OFDM symbols (e.g., the first OFDM symbol) of the corresponding subframe for a physical downlink control channel (PDCCH), that is, an L1/L2 control channel. A Transmission Time Interval (TTI) is a unit time for subframe transmission.

The RRC state of UE and an RRC connection method are described below.

The RRC state means whether or not the RRC layer of UE is logically connected to the RRC layer of the E-UTRAN. A case where the RRC layer of UE is logically connected to the RRC layer of the E-UTRAN is referred to as an RRC connected state. A case where the RRC layer of UE is not logically connected to the RRC layer of the E-UTRAN is referred to as an RRC idle state. The E-UTRAN may check the existence of corresponding UE in the RRC connected state in each cell because the UE has RRC connection, so the UE may be effectively controlled. In contrast, the E-UTRAN is unable to check UE in the RRC idle state, and a Core Network (CN) manages UE in the RRC idle state in each tracking area, that is, the unit of an area greater than a cell. That is, the existence or non-existence of UE in the RRC idle state is checked only for each large area. Accordingly, the UE needs to shift to the RRC connected state in order to be provided with common mobile communication service, such as voice or data.

When a user first powers UE, the UE first searches for a proper cell and remains in the RRC idle state in the corresponding cell. The UE in the RRC idle state establishes RRC connection with an E-UTRAN through an RRC connection procedure when it is necessary to set up the RRC connection, and shifts to the RRC connected state. A case where UE in the RRC idle state needs to set up RRC connection includes several cases. For example, the cases may include a need to send uplink data for a reason, such as a call attempt by a user, and to send a response message as a response to a paging message received from an E-UTRAN.

A Non-Access Stratum (NAS) layer placed over the RRC layer performs functions, such as session management and mobility management.

In the NAS layer, in order to manage the mobility of UE, two types of states: EPS Mobility Management-REGISTERED (EMM-REGISTERED) and EMM-DEREGISTERED are defined. The two states are applied to UE and the MME. UE is initially in the EMM-DEREGISTERED state. In order to access a network, the UE performs a procedure of registering it with the corresponding network through an initial attach procedure. If the attach procedure is successfully performed, the UE and the MME become the EMM-REGISTERED state.

In order to manage signaling connection between UE and the EPC, two types of states: an EPS Connection Management (ECM)-IDLE state and an ECM-CONNECTED state are defined. The two states are applied to UE and the MME. When the UE in the ECM-IDLE state establishes RRC connection with the E-UTRAN, the UE becomes the ECM-CONNECTED state. The MME in the ECM-IDLE state becomes the ECM-CONNECTED state when it establishes S1 connection with the E-UTRAN. When the UE is in the ECM-IDLE state, the E-UTRAN does not have information about the context of the UE. Accordingly, the UE in the ECM-IDLE state performs procedures related to UE-based mobility, such as cell selection or cell reselection, without a need to receive a command from a network. In contrast, when the UE is in the ECM-CONNECTED state, the mobility of the UE is managed in response to a command from a network. If the location of the UE in the ECM-IDLE state is different from a location known to the network, the UE informs the network of its corresponding location through a tracking area update procedure.

System information is described below.

System information includes essential information that needs to be known by UE in order for the UE to access a BS. Accordingly, the UE needs to have received all pieces of system information before accessing the BS, and needs to always have the up-to-date system information. Furthermore, the BS periodically transmits the system information because the system information is information that needs to be known by all UEs within one cell. The system information is divided into a Master Information Block (MIB) and a plurality of System Information Blocks (SIBs).

The MIB may include a limited number of parameters that are most essential and most frequently transmitted when other information is required to be obtained from a cell. UE first searches for an MIB after downlink synchronization. The MIB may include information, such as an SFN that supports downlink channel bandwidth, a PHICH configuration, and synchronization and operates as a timing criterion and an eNB transmit antenna configuration. The MIB may be transmitted on a broadcast channel (BCH) through broadcasting.

SystemInformationBlockType1 (SIB1) of included SIBs is included in a “SystemInformationBlockType1” message and transmitted. The remaining SIBs other than the SIB1 is included in a system information message and transmitted. To map the SIBs to the system information message may be flexibly configured by a scheduling information list parameter included in the SIB1. In this case, each of the SIBs is included in a single system information message, and only SIBs having the same scheduling requirement value (e.g. cycle) may be mapped to the same system information message. Furthermore, a SystemInformationBlockType2 (SIB2) is always mapped to a system information message corresponding to the first entry within the system information message list of a scheduling information list. A plurality of system information messages may be transmitted within the same cycle. The SIB1 and all the system information messages are transmitted on a DL-SCH.

In addition to broadcast transmission, in an E-UTRAN, the SIB1 may be dedicated-signaled in the state in which it includes a parameter configured like an existing configured value. In this case, the SIB1 may be included in an RRC connection reconfiguration message and transmitted.

The SIB1 includes information related to UE cell access, and defines the scheduling of other SIBs. The SIB1 may include information related to the PLMN identifiers of a network, tracking area code (TAC) and a cell ID, a cell barring status indicative of whether a cell is a cell on which camp-on is possible, the lowest reception level required within a cell which is used as cell reselection criterion, and the transmission time and cycle of other SIBs.

The SIB2 may include radio resource configuration information common to all pieces of UE. The SIB2 may include information related to an uplink carrier frequency and uplink channel bandwidth, an RACH configuration, a page configuration, an uplink power control configuration, a sounding reference signal configuration, a PUCCH configuration supporting ACK/NACK transmission, and a PUSCH configuration.

UE may apply a procedure for obtaining system information and detecting a change of system information to a primary cell (PCell) only. In a secondary cell (SCell), when a corresponding SCell is added, an E-UTRAN may provide all of pieces of system information related to an RRC connection state operation through dedicated signaling. When system information related to a configured SCell is changed, an E-UTRAN may release an SCell that is taken into consideration and subsequently add the changed system information. This may be performed along with a single RRC connection reconfiguration message. An E-UTRAN may configure parameter values different from a value broadcasted within an SCell that has been taken into consideration through dedicated signaling.

UE needs to guarantee the validity of a specific type of system information, and such system information is called required system information. The required system information may be defined as follows.

-   -   If UE is an RRC idle state: The UE needs to be guaranteed so         that it has the valid versions of the MIB and the SIB1 in         addition to the SIB2 to SIB8. This may comply with the support         of a radio access technology (RAT) that is taken into         consideration.     -   If UE is an RRC connection state: The UE needs to be guaranteed         so that it has the valid versions of the MIB, the SIB1, and the         SIB2.

In general, the validity of system information may be guaranteed up to a maximum of 3 hours after the system information is obtained.

In general, service that is provided to UE by a network may be classified into three types as follows. Furthermore, the UE differently recognizes the type of cell depending on what service may be provided to the UE. In the following description, a service type is first described, and the type of cell is described.

1) Limited service: this service provides emergency calls and an Earthquake and Tsunami Warning System (ETWS), and may be provided by an acceptable cell.

2) Suitable service: this service means public service for common uses, and may be provided by a suitable cell (or a normal cell).

3) Operator service: this service means service for communication network operators. This cell may be used by only communication network operators, but may not be used by common users.

In relation to a service type provided by a cell, the type of cell may be classified as follows.

1) An acceptable cell: this cell is a cell from which UE may be provided with limited service. This cell is a cell that has not been barred from a viewpoint of corresponding UE and that satisfies the cell selection criterion of the UE.

2) A suitable cell: this cell is a cell from which UE may be provided with suitable service. This cell satisfies the conditions of an acceptable cell and also satisfies additional conditions. The additional conditions include that the suitable cell needs to belong to a Public Land Mobile Network (PLMN) to which corresponding UE may access and that the suitable cell is a cell on which the execution of a tracking area update procedure by the UE is not barred. If a corresponding cell is a CSG cell, the cell needs to be a cell to which UE may access as a member of the CSG.

3) A barred cell: this cell is a cell that broadcasts information indicative of a barred cell through system information.

4) A reserved cell: this cell is a cell that broadcasts information indicative of a reserved cell through system information.

FIG. 4 is a flowchart illustrating the operation of UE in the RRC idle state. FIG. 4 illustrates a procedure in which UE that is initially powered on experiences a cell selection procedure, registers it with a network, and then performs cell reselection if necessary.

Referring to FIG. 4, the UE selects Radio Access Technology (RAT) in which the UE communicates with a Public Land Mobile Network (PLMN), that is, a network from which the UE is provided with service (S410). Information about the PLMN and the RAT may be selected by the user of the UE, and the information stored in a Universal Subscriber Identity Module (USIM) may be used.

The UE selects a cell that has the greatest value and that belongs to cells having measured BS and signal intensity or quality greater than a specific value (cell selection) (S420). In this case, the UE that is powered off performs cell selection, which may be called initial cell selection. A cell selection procedure is described later in detail. After the cell selection, the UE receives system information periodically by the BS. The specific value refers to a value that is defined in a system in order for the quality of a physical signal in data transmission/reception to be guaranteed. Accordingly, the specific value may differ depending on applied RAT.

If network registration is necessary, the UE performs a network registration procedure (S430). The UE registers its information (e.g., an IMSI) with the network in order to receive service (e.g., paging) from the network. The UE does not register it with a network whenever it selects a cell, but registers it with a network when information about the network (e.g., a Tracking Area Identity (TAI)) included in system information is different from information about the network that is known to the UE.

The UE performs cell reselection based on a service environment provided by the cell or the environment of the UE (S440). If the value of the intensity or quality of a signal measured based on a BS from which the UE is provided with service is lower than that measured based on a BS of a neighboring cell, the UE selects a cell that belongs to other cells and that provides better signal characteristics than the cell of the BS that is accessed by the UE. This procedure is called cell reselection differently from the initial cell selection of the No. 2 procedure. In this case, temporal restriction conditions are placed in order for a cell to be frequently reselected in response to a change of signal characteristic. A cell reselection procedure is described later in detail.

FIG. 5 is a flowchart illustrating a procedure of establishing RRC connection.

UE sends an RRC connection request message that requests RRC connection to a network (S510). The network sends an RRC connection establishment message as a response to the RRC connection request (S520). After receiving the RRC connection establishment message, the UE enters RRC connected mode.

The UE sends an RRC connection establishment complete message used to check the successful completion of the RRC connection to the network (S530).

FIG. 6 is a flowchart illustrating an RRC connection reconfiguration procedure. An RRC connection reconfiguration is used to modify RRC connection. This is used to establish/modify/release RBs, perform handover, and set up/modify/release measurements.

A network sends an RRC connection reconfiguration message for modifying RRC connection to UE (S610). As a response to the RRC connection reconfiguration message, the UE sends an RRC connection reconfiguration complete message used to check the successful completion of the RRC connection reconfiguration to the network (S620).

Hereinafter, a public land mobile network (PLMN) is described.

The PLMN is a network which is disposed and operated by a mobile network operator. Each mobile network operator operates one or more PLMNs. Each PLMN may be identified by a Mobile Country Code (MCC) and a Mobile Network Code (MNC). PLMN information of a cell is included in system information and broadcasted.

In PLMN selection, cell selection, and cell reselection, various types of PLMNs may be considered by the terminal.

Home PLMN (HPLMN): PLMN having MCC and MNC matching with MCC and MNC of a terminal IMSI.

Equivalent HPLMN (EHPLMN): PLMN serving as an equivalent of an HPLMN.

Registered PLMN (RPLMN): PLMN successfully finishing location registration.

Equivalent PLMN (EPLMN): PLMN serving as an equivalent of an RPLMN.

Each mobile service consumer subscribes in the HPLMN. When a general service is provided to the terminal through the HPLMN or the EHPLMN, the terminal is not in a roaming state. Meanwhile, when the service is provided to the terminal through a PLMN except for the HPLMN/EHPLMN, the terminal is in the roaming state. In this case, the PLMN refers to a Visited PLMN (VPLMN).

When UE is initially powered on, the UE searches for available Public Land Mobile Networks (PLMNs) and selects a proper PLMN from which the UE is able to be provided with service. The PLMN is a network that is deployed or operated by a mobile network operator. Each mobile network operator operates one or more PLMNs. Each PLMN may be identified by Mobile Country Code (MCC) and Mobile Network Code (MNC). Information about the PLMN of a cell is included in system information and broadcasted. The UE attempts to register it with the selected PLMN. If registration is successful, the selected PLMN becomes a Registered PLMN (RPLMN). The network may signalize a PLMN list to the UE. In this case, PLMNs included in the PLMN list may be considered to be PLMNs, such as RPLMNs. The UE registered with the network needs to be able to be always reachable by the network. If the UE is in the ECM-CONNECTED state (identically the RRC connection state), the network recognizes that the UE is being provided with service. If the UE is in the ECM-IDLE state (identically the RRC idle state), however, the situation of the UE is not valid in an eNB, but is stored in the MME. In such a case, only the MME is informed of the location of the UE in the ECM-IDLE state through the granularity of the list of Tracking Areas (TAs). A single TA is identified by a Tracking Area Identity (TAI) formed of the identifier of a PLMN to which the TA belongs and Tracking Area Code (TAC) that uniquely expresses the TA within the PLMN.

Thereafter, the UE selects a cell that belongs to cells provided by the selected PLMN and that has signal quality and characteristics on which the UE is able to be provided with proper service.

The following is a detailed description of a procedure of selecting a cell by a terminal.

When power is turned-on or the terminal is located in a cell, the terminal performs procedures for receiving a service by selecting/reselecting a suitable quality cell.

A terminal in an RRC idle state should prepare to receive a service through the cell by always selecting a suitable quality cell. For example, a terminal where power is turned-on just before should select the suitable quality cell to be registered in a network. If the terminal in an RRC connection state enters in an RRC idle state, the terminal should selects a cell for stay in the RRC idle state. In this way, a procedure of selecting a cell satisfying a certain condition by the terminal in order to be in a service idle state such as the RRC idle state refers to cell selection. Since the cell selection is performed in a state that a cell in the RRC idle state is not currently determined, it is important to select the cell as rapid as possible. Accordingly, if the cell provides a wireless signal quality of a predetermined level or greater, although the cell does not provide the best wireless signal quality, the cell may be selected during a cell selection procedure of the terminal.

A method and a procedure of selecting a cell by a terminal in a 3GPP LTE is described with reference to 3GPP TS 36.304 V8.5.0 (2009-03) “User Equipment (UE) procedures in idle mode (Release 8)”.

A cell selection procedure is basically divided into two types.

The first is an initial cell selection procedure. In this procedure, UE does not have preliminary information about a wireless channel. Accordingly, the UE searches for all wireless channels in order to find out a proper cell. The UE searches for the strongest cell in each channel. Thereafter, if the UE has only to search for a suitable cell that satisfies a cell selection criterion, the UE selects the corresponding cell.

Next, the UE may select the cell using stored information or using information broadcasted by the cell. Accordingly, cell selection may be fast compared to an initial cell selection procedure. If the UE has only to search for a cell that satisfies the cell selection criterion, the UE selects the corresponding cell. If a suitable cell that satisfies the cell selection criterion is not retrieved though such a procedure, the UE performs an initial cell selection procedure.

A cell selection criterion may be defined as in Equation 1 below.

Srxlev>0 AND Squal>0.  [Equation 1]

-   -   where:

Srxlev=Q _(rxlevmeas)−(Q _(rxlevmin) +Q _(rxlevminoffset))−P _(compensation),

Squal=Q _(qualmeas)−(Q _(qualmin) +Q _(qualminoffset))

In this case, in Equation 1, the variables may be defined as in Table 1 below.

TABLE 1 Srxlev Cell selection RX level value (dB) Squal Cell selection quality value (dB) Q_(rxlevmeas) Measured cell RX level value (RSRP) Q_(qualmeas) Measured cell quality value (RSRQ) Q_(rxlevmin) Minimum required RX level in the cell (dBm) Q_(qualmin) Minimum required quality level in the cell (dB) Q_(rxlevminoffset) Offset to the signalled Q_(rxlevmin) taken into account in the Srxlev evaluation as a result of a periodic search for a higher priority PLMN while camped normally in a VPLMN Q_(qualminoffset) Offset to the signalled Q_(qualmin) taken into account in the Squal evaluation as a result of a periodic search for a higher priority PLMN while camped normally in a VPLMN Pcompensation max(P_(EMAX) − P_(PowerClass), 0) (dB) P_(EMAX) Maximum TX power level an UE may use when transmitting on the uplink in the cell (dBm) defined as P_(EMAX) in [TS 36.101] P_(PowerClass) Maximum RF output power of the UE (dBm) according to the UE power class as defined in [TS 36.101]

Qrxlevminoffset and Qqualminoffset, that is, signaled values, are the results of periodic discovery for a PLMN having higher priority while UE camps on a normal cell within a VPLMN, and may be applied only when cell selection is evaluated. As described above, during the periodic discovery of a PLMN having higher priority, UE may perform cell selection evaluation using parameter values stored from another cell of the PLMN having such higher priority.

After UE selects any cell through a cell selection procedure, the intensity or quality of a signal between the UE and a BS may be changed due to the mobility of the UE or a change of a radio environment. Accordingly, if the quality of the selected cell is changed, the UE may select another cell providing better quality.

After the UE selects a specific cell through the cell selection procedure, the intensity or quality of a signal between the UE and a BS may be changed due to a change in the mobility or wireless environment of the UE. Accordingly, if the quality of the selected cell is deteriorated, the UE may select another cell that provides better quality. If a cell is reselected as described above, the UE selects a cell that provides better signal quality than the currently selected cell. Such a procedure is called cell reselection. In general, a basic object of the cell reselection procedure is to select a cell that provides UE with the best quality from a viewpoint of the quality of a radio signal.

In addition to the viewpoint of the quality of a radio signal, a network may determine priority corresponding to each frequency, and may inform the UE of the determined priorities. The UE that has received the priorities preferentially takes into consideration the priorities in a cell reselection procedure compared to a radio signal quality criterion.

As described above, there is a method of selecting or reselecting a cell according to the signal characteristics of a wireless environment. In selecting a cell for reselection when a cell is reselected, the following cell reselection methods may be present according to the RAT and frequency characteristics of the cell.

-   -   Intra-frequency cell reselection: UE reselects a cell having the         same center frequency as that of RAT, such as a cell on which         the UE camps on.     -   Inter-frequency cell reselection: UE reselects a cell having a         different center frequency from that of RAT, such as a cell on         which the UE camps on     -   Inter-RAT cell reselection: UE reselects a cell that uses RAT         different from RAT on which the UE camps

The principle of a cell reselection procedure is as follows.

First, UE measures the quality of a serving cell and neighbor cells for cell reselection.

Second, cell reselection is performed based on a cell reselection criterion. The cell reselection criterion has the following characteristics in relation to the measurements of a serving cell and neighbor cells.

Intra-frequency cell reselection is basically based on ranking. Ranking is a task for defining a criterion value for evaluating cell reselection and numbering cells using criterion values according to the size of the criterion values. A cell having the best criterion is commonly called the best-ranked cell. The cell criterion value is based on the value of a corresponding cell measured by UE, and may be a value to which a frequency offset or cell offset has been applied, if necessary.

Inter-frequency cell reselection is based on frequency priority provided by a network. UE attempts to camp on a frequency having the highest frequency priority. A network may provide frequency priority that will be applied by UEs within a cell in common through broadcasting signaling, or may provide frequency-specific priority to each UE through UE-dedicated signaling. A cell reselection priority provided through broadcast signaling may refer to a common priority. A cell reselection priority for each terminal set by a network may refer to a dedicated priority. If receiving the dedicated priority, the terminal may receive a valid time associated with the dedicated priority together. If receiving the dedicated priority, the terminal starts a validity timer set as the received valid time together therewith. While the valid timer is operated, the terminal applies the dedicated priority in the RRC idle mode. If the valid timer is expired, the terminal discards the dedicated priority and again applies the common priority.

For the inter-frequency cell reselection, a network may provide UE with a parameter (e.g., a frequency-specific offset) used in cell reselection for each frequency. For the intra-frequency cell reselection or the inter-frequency cell reselection, a network may provide UE with a Neighboring Cell List (NCL) used in cell reselection. The NCL includes a cell-specific parameter (e.g., a cell-specific offset) used in cell reselection. For the intra-frequency or inter-frequency cell reselection, a network may provide UE with a cell reselection black list used in cell reselection.

The UE does not perform cell reselection on a cell included in the black list.

Ranking performed in a cell reselection evaluation procedure is described below.

A ranking criterion used to give the priority of a cell is defined as in Equation 2.

R _(s) =Q _(meas,s) +Q _(hyst) ,R _(n) =Q _(meas,n) −Q _(offset)  [Equation 2]

In Equation 2, Rs is the ranking criterion of a serving cell on which UE now camps, Rn is the ranking criterion of a neighboring cell, Qmeas,s is the quality value of the serving cell measured by the UE, Qmeas,n is the quality value of the neighboring cell measured by the UE, Qhyst is a hysteresis value for ranking, and Qoffset is an offset between the two cells.

In Intra-frequency, if UE receives an offset “Qoffsets,n” between a serving cell and a neighbor cell, Qoffset=Qoffsets,n. If UE does not Qoffsets,n, Qoffset=0.

In Inter-frequency, if UE receives an offset “Qoffsets,n” for a corresponding cell, Qoffset=Qoffsets,n+Qfrequency. If UE does not receive “Qoffsets,n”, Qoffset=Qfrequency.

If the ranking criterion Rs of a serving cell and the ranking criterion Rn of a neighbor cell are changed in a similar state, ranking priority is frequency changed as a result of the change, and UE may alternately reselect the twos. Qhyst is a parameter that gives hysteresis to cell reselection so that UE is prevented from to alternately reselecting two cells.

UE measures RS of a serving cell and Rn of a neighbor cell according to the above equation, considers a cell having the greatest ranking criterion value to be the best-ranked cell, and reselects the cell.

In accordance with the criterion, it may be checked that the quality of a cell is the most important criterion in cell reselection. If a reselected cell is not a suitable cell, UE excludes a corresponding frequency or a corresponding cell from the subject of cell reselection.

Hereinafter, radio link failure (RLF) will be described.

UE continues to perform measurements in order to maintain the quality of a radio link with a serving cell from which the UE receives service. The UE determines whether or not communication is impossible in a current situation due to the deterioration of the quality of the radio link with the serving cell. If communication is almost impossible because the quality of the serving cell is too low, the UE determines the current situation to be an RLF.

If the RLF is determined, the UE abandons maintaining communication with the current serving cell, selects a new cell through cell selection (or cell reselection) procedure, and attempts RRC connection re-establishment with the new cell.

In the specification of 3GPP LTE, the following examples are taken as cases where normal communication is impossible.

-   -   A case where UE determines that there is a serious problem in         the quality of a downlink communication link (a case where the         quality of a PCell is determined to be low while performing RLM)         based on the radio quality measured results of the PHY layer of         the UE     -   A case where uplink transmission is problematic because a random         access procedure continues to fail in the MAC sublayer.     -   A case where uplink transmission is problematic because uplink         data transmission continues to fail in the RLC sublayer.     -   A case where handover is determined to have failed.     -   A case where a message received by UE does not pass through an         integrity check.

An RRC connection re-establishment procedure is described in more detail below.

FIG. 7 is a diagram illustrating an RRC connection re-establishment procedure.

Referring to FIG. 7, UE stops using all the radio bearers that have been configured other than a Signaling Radio Bearer (SRB) #0, and initializes a variety of kinds of sublayers of an Access Stratum (AS) (S710). Furthermore, the UE configures each sublayer and the PHY layer as a default configuration. In this procedure, the UE maintains the RRC connection state.

The UE performs a cell selection procedure for performing an RRC connection reconfiguration procedure (S720). The cell selection procedure of the RRC connection re-establishment procedure may be performed in the same manner as the cell selection procedure that is performed by the UE in the RRC idle state, although the UE maintains the RRC connection state.

After performing the cell selection procedure, the UE determines whether or not a corresponding cell is a suitable cell by checking the system information of the corresponding cell (S730). If the selected cell is determined to be a suitable E-UTRAN cell, the UE sends an RRC connection re-establishment request message to the corresponding cell (S740).

Meanwhile, if the selected cell is determined to be a cell that uses RAT different from that of the E-UTRAN through the cell selection procedure for performing the RRC connection re-establishment procedure, the UE stops the RRC connection re-establishment procedure and enters the RRC idle state (S750).

The UE may be implemented to finish checking whether the selected cell is a suitable cell through the cell selection procedure and the reception of the system information of the selected cell. To this end, the UE may drive a timer when the RRC connection re-establishment procedure is started. The timer may be stopped if it is determined that the UE has selected a suitable cell. If the timer expires, the UE may consider that the RRC connection re-establishment procedure has failed, and may enter the RRC idle state. Such a timer is hereinafter called an RLF timer. In LTE spec TS 36.331, a timer named “T311” may be used as an RLF timer. The UE may obtain the set value of the timer from the system information of the serving cell.

If an RRC connection re-establishment request message is received from the UE and the request is accepted, a cell sends an RRC connection re-establishment message to the UE.

The UE that has received the RRC connection re-establishment message from the cell reconfigures a PDCP sublayer and an RLC sublayer with an SRB1. Furthermore, the UE calculates various key values related to security setting, and reconfigures a PDCP sublayer responsible for security as the newly calculated security key values. Accordingly, the SRB 1 between the UE and the cell is open, and the UE and the cell may exchange RRC control messages. The UE completes the restart of the SRB1, and sends an RRC connection re-establishment complete message indicative of that the RRC connection re-establishment procedure has been completed to the cell (S760).

In contrast, if the RRC connection re-establishment request message is received from the UE and the request is not accepted, the cell sends an RRC connection re-establishment reject message to the UE.

If the RRC connection re-establishment procedure is successfully performed, the cell and the UE perform an RRC connection reconfiguration procedure. Accordingly, the UE recovers the state prior to the execution of the RRC connection re-establishment procedure, and the continuity of service is guaranteed to the upmost.

FIG. 8 illustrates sub states where the terminal may have in an RRC_IDLE state and a sub state transition process.

Referring to FIG. 8, a terminal performs an initial cell selection process (S801). The initial cell selection process may be performed when there is no stored cell information with respect to the PLMN or a suitable cell is not found.

If the suitable cell is not found in the initial cell selection process, the terminal transitions to an any cell selection state (S802). The optional cell selection state represents a state which does not camp on in both of a suitable cell and an acceptable cell. The optional cell selection state is a state attempted by the terminal in order to find an acceptable cell of an optional PLMN which may camp on. When the terminal finds no cells which may camp on, the terminal is continuously maintained in an optional cell selection state until the acceptable cell is found.

If the suitable cell is found in the initial cell selection process, the state transits to a normal camp state (S803). The normal camp state represents a state which camps on the normal cell. A paging channel is selected according to information given through system information to motor, and an evaluation process for cell reselection may be performed.

In the normal camp state (S803), if a cell reselection evaluation process (S804) is caused, the cell reselection evaluation process (S804) is performed. If a suitable cell is found in the cell reselection evaluation process (S804), the terminal again transits to the normal camp state (S803).

If an acceptable cell is found in the any cell selection state (S802), the terminal transits to an any cell camped state (S805). The any cell camped state (S805) represents a state of camping on an acceptable cell.

In the any cell camped state (S805), the terminal may select a paging channel according to information given through system information to monitor, and may perform a cell reselection evaluation process (S806). If the acceptable cell is not found in the cell reselection evaluation process (S806), the terminal transits the any cell selection state (S802).

Hereinafter, a D2D operation will be described. In the 3GPP LTE-A, a service related to the D2D operation refers to Proximity based Services (ProSe). Hereinafter, the ProSe is an equivalent concept with the D2D operation and the ProSe may be compatibly used with the D2D operation. The ProSe is now described.

The ProSe includes ProSe direct communication and ProSe direct discovery. The ProSe direct communication presents communication performed by two or more adjacent terminals. The terminals may perform communication using a protocol of a user plane. A ProSe-enabled UE means a UE for supporting a process related to requirements of the ProSe. Unless otherwise defined, the ProSe-enabled UE includes both of a public safety UE and a non-public safety UE. The public safety UE represents a UE for supporting both of a public safety specified function and the ProSe process. The non-public safety UE is a terminal which supports the ProSe process but does not support the public safety specified function.

The ProSe direct discovery is a process where the ProSe-enabled UE discovers another ProSe-enabled UE. In this case, only ability of the two ProSe-enabled UEs is used. An EPC-level ProSe discovery signifies a process where an EPC determines whether 2 ProSe enable terminals are closed to each other, and reports the close state thereof the two ProSe enabled terminals.

Hereinafter, the ProSe direct communication may refer to D2D communication, and the ProSe direct discovery may refer to D2D discovery.

FIG. 9 illustrates a reference structure for a ProSe.

Referring to FIG. 9, the reference structure for a ProSe includes a plurality of terminals having E-UTRAN, EPC, and ProSe application program, a ProSe application (APP) server, and a ProSe function.

An EPC is a representative example of the E-UTRAN. The EPC may include an MME, an S-GW, a P-GW, a policy and charging rules function (PCRF), and a home subscriber server (HSS).

The ProSe application server is a user of ProSe in order to make an application function. The ProSe application server may communicate with an application program in the terminal. The application program in the terminal may use a ProSe ability to make an application function.

The ProSe function may include at least one of following functions but is not limited thereto.

-   -   Interworking via a reference point towards the 3rd party         applications     -   Authorization and configuration of the UE for discovery and         direct communication)     -   Enable the function of the EPC level ProSe discovery     -   ProSe related new subscriber data and handling of data storage,         and also handling of ProSe identities     -   Security related function     -   Provide control towards the EPC for policy related function     -   Provide function for charging (via or outside of EPC, e.g.,         offline charging))

Hereinafter, a reference point and a reference interface will be described in a reference structure for the ProSe.

-   -   PC1: a reference point between a ProSe application program in         the terminal and a ProSe application program in a ProSe         application server. The PC1 is used to define signaling         requirements in an application level.     -   PC2: is a reference point between the ProSe application server         and a ProSe function. The PC2 is used to define an interaction         between the ProSe application server and a ProSe function. An         application data update of a ProSe database of the ProSe         function may be an example of the interaction.     -   PC3: is a reference point between the terminal and the ProSe         function. The PC3 is used to define an interaction between the         terminal and the ProSe function. Configuration for ProSe         discovery and communication may be an example of the         interaction.     -   PC4: is a reference point between an EPC and the ProSe function.         The PC4 is used to define an interaction between the EPC and the         ProSe function. The interaction lay illustrate when a path for         1:1 communication or a ProSe service for real time session         management or mobility management are authorized.     -   PC5: is a reference point to use control/user plane for         discovery, communication, and relay between terminals, and 1:1         communication.     -   PC6: is a reference point to use a function such as ProSe         discovery between users included in different PLMNs.     -   SGi: may be used for application data and application level         control information exchange.

<ProSe Direct Communication (D2D Communication)>.

The ProSe direct communication is a communication mode where two public safety terminals may perform direct communication through a PC 5 interface. The communication mode may be supported in both of a case of receiving a service in coverage of E-UTRAN or a case of separating the coverage of E-UTRAN.

FIG. 10 illustrates arrangement examples of terminals performing ProSe direct communication and cell coverage.

Referring to FIG. 10(a), UEs A and B may be located outside of the cell coverage. Referring to FIG. 10(b), the UE A may be located in the cell coverage and the UE B may be located outside of the cell coverage. Referring to FIG. 10(c), both of UEs A and B may be located in the cell coverage. Referring to FIG. 10(d), the UE A may be located in coverage of a first cell and the UE B may be in coverage of a second cell.

As described above, the ProSe direct communication may be performed between terminals which are provided at various positions.

Meanwhile, following IDs may be used in the ProSe direct communication.

Source layer-2 ID: The source layer-2 ID identifies a sender of a packet in a PC 5 interface.

Purpose layer-2 ID: The purpose layer-2 ID identifies a target of a packet in a PC 5 interface.

SA L1 ID: The SA L1 ID represents an in an ID in a scheduling assignment (SA) in the PC 5 interface.

FIG. 11 illustrates a user plane protocol stack for the ProSe direct communication.

Referring to FIG. 11, the PC 5 interface includes a PDCH layer, a RLC layer, a MAC layer, and a PHY layer.

There may not be HARQ feedback in the ProSe direct communication. An MAC header may include the source layer-2 ID and the purpose layer-2 ID.

<Radio Resource Assignment for ProSe Direct Communication>.

A ProSe enable terminal may use following two modes with respect to resource assignments for the ProSe direct communication.

1. Mode 1

The mode 2 is a mode for receiving scheduling a resource for the ProSe direct communication from a base station. The terminal should be in a RRC_CONNECTED state according to the mode 1 in order to transmit data. The terminal requests a transmission resource to the base station, and the base station schedules a resource for scheduling assignment and data transmission. The terminal may transmit a scheduling request to the base station and may transmit a Buffer Status Report (ProSe BSR). The base station has data which the terminal will perform the ProSe direct communication and determines whether a resource for transmitting the data is required.

2. Mode 2

The mode 2 is a mode for selecting a direct resource. The terminal directly selects a resource for the ProSe direct communication from a resource pool. The resource pool may be configured by a network or may be previously determined.

Meanwhile, when the terminal includes a serving cell, that is, when the terminal is in an RRC_CONNECTED state with the base station or is located in a specific cell in an RRC_IDLE state, the terminal is regarded to be in coverage of the base station.

If the terminal is located outside of the coverage, only the mode 2 is applicable. If the terminal is located in the coverage, the mode 1 or the mode 2 may be used according to setting of the base station.

If there are no exceptional conditions, only when the base station is configured, the terminal may change a mode from the mode 1 to the mode 2 or from the mode 2 to the mode 1.

<ProSe Direct Discovery (D2D Discovery)>

The ProSe direct discovery represents a process used to discover when the ProSe enabled terminal discovers other neighboring ProSe enabled terminal and refers to D2D direction discovery or D2D discovery. In this case, an E-UTRA wireless signal through the PC 4 interface may be used. Hereinafter, information used for the ProSe direct discovery refers to discovery information.

FIG. 12 illustrates a PC 5 interface for D2D discovery.

Referring to FIG. 12, the PC 5 interface includes an MAC layer, a PHY layer, and a ProSe Protocol layer being an upper layer. Permission for announcement and monitoring of discovery information is handled in the upper layer ProSe Protocol. Contents of discovery information are transparent to an access stratum (AS). The ProSe Protocol allows only valid discovery information to be transferred to the AS for announcement.

An MAC layer receives discovery information from the upper layer ProSe Protocol. An IP layer is not used for transmitting the discovery information. The MAC layer determines a resource used in order to announce the discovery information received from the upper layer. The MAC layer makes and sends a protocol data unit (MAC PDU) to a physical layer. An MAC header is not added.

There are two types of resource assignments for announcing the discovery information.

1. Type 1

The type 1 is a method assigned so that resources for announcing the discovery information are not terminal-specific and the base station provides resource pool configuration for announcing the discovery information to the terminals. The configuration may be included in a system information block (SIB) to be signaled in a broadcast scheme. Alternatively, the configuration may be included in a terminal specific RRC message to be provided. Alternatively, the configuration may be broadcast-signaled or terminal-specific signaled of a different layer from the RRC message.

The terminal selects a resource from an indicated resource pool to announce discovery information using the selected resource. The terminal may announce discovery information through a resource optionally selected during each discovery period.

2. Type 2

The type 2 is a method where resources for announcing the discovery information are terminal-specifically assigned. A terminal in a RRC_CONNECTED state may request a resource for announcing a discovery signal to the base station through a RRC signal. The base station may assign a resource for announcing a discovery signal as an RRC signal. A resource for monitoring the discovery signal in a configured resource pool may be assigned in terminals.

With respect to a terminal in an RRC_IDLE state, a base station may report a type 1 resource pool for announcing the discovery signal as an SIB. Terminals where ProSe direct discovery is allowed use a type 1 resource pool for announcing the discovery information in the RRC_IDLE state. Alternatively, the base station 2) reports that the base station supports the ProSe direct discovery through the SIB but may not provide the resource for announcing the discovery information. In this case, the terminal should enter the RRC_CONNECTED state for announcing the discovery information.

With respect to a terminal in an RRC_CONNECTED state, the base station may configure whether to use a type 1 resource pool or a type 2 resource pool for announcing the discovery information through a RRC signal.

FIG. 13 illustrates an embodiment of a ProSe direct discovery procedure.

Referring to FIG. 13, it is assumed in a terminal A and a terminal B that a ProSe-enabled application program is operated, and the terminal A and the terminal B are configured in a friend relationship to each other, that is, a relationship capable of allowing D2D communication with each other in the application program. Hereinafter, the terminal B may be expressed as a friend of the terminal A. For example, the application program may be a social networking program. 3GPP Layers correspond to functions of an application program for using a ProSe discovery service regulated according to 3GPP.

A ProSe direct discovery between the terminal A and the terminal B may perform a following procedure.

1. First, the terminal A performs regular application-Layer communication with an application server. The above communication is performed based on Application programming interface (API).

2. A ProSe enabled application program of the terminal A receives a list of application layer IDs having a friend relationship. The application layer ID may generally be in the form of a network access ID. For example, an application layer ID of the terminal A may have a form such as adam@example.com.

3. A terminal A requests private expression codes for a user and private expression codes for a friend of the user.

4. 3GPP layers transmit an expression code request to a ProSe server.

5. The ProSe server map application layer IDs provided from an operator or a third application server to private expression codes. For example, an application layer ID such as adam@example.com. The mapping may be performed based on parameters (e.g., mapping algorithms, key values, and the like) received from an application service of the network.

6. The ProSe server responds the obtained expression codes to the 3GPP layers. The 3GPP layers report that expression codes with respect to the requested application layer are successively received to the ProSe enabled application program. Further, a mapping table between the application layer IDs and the expression codes are generated.

7. The ProSe enabled application program requests the 3GPP layers to start the discovery procedure. That is, when one of friends is located close to the terminal A and direct communication may be performed, the ProSe enabled application program attempts the discovery. 3GPP layers announce a private expression code of the terminal A (that is, “GTER543$#2FSJ67DFSF” which is a private expression code of adam@example.com in the above example). In mapping of an application layer ID of a corresponding application program and the private expression code, the mapping relationship may be known by the previously received friends, and the mapping may be performed.

8. It is assumed that the terminal B is operating the same ProSe enabled application program as that of the terminal A, and the above steps 3 to 6 may be executed. 3GPP layers included in the terminal B may perform ProSe discovery.

9. When the terminal B receives the above announce from the terminal A, the terminal B determines whether the private expression code included in the announce is known by the terminal B or is mapped to an application layer ID. As illustrated in step 8, since the terminal B performs steps 3 to 6, the terminal B knows a private expression code with respect to the terminal A, mapping of the private expression code to the application layer ID, and which is a corresponding application program. Accordingly, the terminal B may discover the terminal B from the announce of the terminal A. The 3GPP layers in the terminal B announces that adam@example.com is discovered to the ProSe enable application program.

FIG. 13 illustrates a discovery procedure by taking into consideration the terminals A and B, the ProSe server, and the application server. Only an operation side between the terminals A and B is described. The terminal A transmits a signal called the announce (the procedure may refer to announcement), and the terminal B receives the announce to discover the terminal A. That is, a discovery procedure of FIG. 13 in an operation directly related to another terminal among operations performed by each terminal may refer to a single step discovery procedure may refer to a single step discovery procedure in a side of one step.

FIG. 14 illustrates another embodiment of a ProSe direct discovery procedure.

In FIG. 14, it is assumed that the terminal 1 to the terminal 4 may be included in a specific group communication system enablers (GCSE) group. It is assumed that the terminal 1 is a discoverer and terminals 2, 3, and 4 are a discoveree. A terminal 5 is a terminal regardless of a discovery procedure.

The terminal 1 and the terminals 2 to 4 may perform a following operation in a discovery procedure.

First, the terminal 1 broadcasts a targeted discovery request message (hereinafter referred to ‘discovery request message’ or ‘M1’) in order to discover whether an optional terminal included in the GCSE group is located around the terminal 1. The targeted discovery request message may include a unique application program group ID or a layer-2 group ID of the specific GCSE group. Further, the targeted discovery request message may include a unique ID of the terminal 1, that is, an application program private ID. The targeted discovery request message may be received by the terminals.

The terminal 5 transmits no response messages. The terminals 2, 3, and 4 included in the GCSE group transmit a targeted discovery response message (hereinafter referred to as a discovery response message or M2) as a response to the targeted discovery request message. The targeted discovery response message may include a unique application program private ID of a terminal transmitting the message.

An operation of terminals in a ProSe discovery procedure illustrated in FIG. 14 will be described. A discoverer (UE 1) transmits the targeted discovery request message, and receives a targeted discovery response message being a response thereto. In addition, if a discoveree (e.g., UE 2) receives the targeted discovery request message, the discoveree transmits a targeted discovery response message as a response thereto. Accordingly, each terminal performs an operation a second step. In the above side, a ProSe discovery procedure of FIG. 14 may refer to a discovery procedure.

In addition to the discovery procedure illustrated in FIG. 14, if the terminal 1 (discoverer) transmits a discovery confirm message (hereinafter may refer to M3) as a response to the targeted discovery response message, this may refer to a third step discovery procedure.

Hereinafter, the present invention will be described. The present invention relates synchronization between network nodes (hereinafter, a terminal will be described as an example but other devices are possible) performing a D2D operation. According to the related art, a terminal performs synchronization using a synchronization signal transmitted from a base station and then transmits/receives the signal. Accordingly, the terminal does not need to perform reporting with respect to a synchronization signal. Meanwhile, the D2D may be performed between terminals. In this case, synchronization should be achieved between terminals performing the D2D operation for transmission and reception/decoding of a wireless signal.

First, a synchronization signal to be used in the D2D operation will be described.

<Synchronization Signal in D2D Operation>

According to the related art, the synchronization signal is transmitted by a center network node (e.g., base station) using a downlink resource. However, in the D2D operation, the synchronization signal may be transmitted by the terminal. In particular, when a synchronization signal transmitted by the base station is not detected or is very weak to be identified, the synchronization signal may be transmitted by the terminal for the D2D operation.

That is, an exact reception/decoding of a wireless signal may be possible by adjusting the synchronization between the terminals. A synchronization signal is a signal used in order to obtain synchronization of a time and a frequency. In the D2D operation, a network node, for example, a terminal instead of a base station may transmit the synchronization signal. Hereinafter, unless defined otherwise, the synchronization means a synchronization signal in the D2D operation, that is, a synchronization signal transmitted from the network node instead of the base station. Further, hereinafter, the synchronization signal may mean a signal having all or a part of following characteristics.

1) It is regarded that the synchronization signal is transmitted by a terminal. 2) When the second terminal receiving the synchronization signal from the first terminal performs synchronization based on the synchronization signal, the first terminal may adjust synchronization for reception of the D2D signal transmitted from the first terminal and reception of a D2D signal transmitted from a third terminal being a different synchronized terminal based on the synchronization signal. 3) The synchronization signal is transmitted through an uplink channel. 4) The synchronization signal is transmitted through uplink resource/uplink sub-frame/uplink frequency. 5) The synchronization signal is transmitted for coexistence with the synchronization signal transmitted from a network.

When the terminal provides a specific sequence (synchronization sequence) to be used as a synchronization reference to another terminal, the terminal may broadcast information including an indicator indicating whether the terminal is located in network coverage. The terminal receiving the specific sequence may determine whether the specific sequence is a synchronization signal used in the network coverage or outside of the network coverage.

Meanwhile, if a terminal receives the synchronization signal at a time t1, the terminal may transmit a synchronization signal at a time interval t1-t2. In this case, the t2 may be a positive value, a negative value, or zero as offset. The t2 may be regulated as a fixe value, may be set by a network, and may be a value induced from PUSCH transmission timing of a cell to which the terminal belongs.

The synchronization signal used for the D2D operation transmitted from a network node (e.g., terminal) except for the base station may transfer an ID and/or a type of a subject transmitting the synchronization signal.

The synchronization signal may include a primary synchronization signal and a secondary synchronization signal. The primary synchronization signal may use a Zadoff Chu sequence and the secondary synchronization signal may use an M sequence. The Zadoff Chu sequence has constant amplitude and is a sequence with zero correlation, and the M sequence is a type of a pseudorandom binary sequence.

Hereinafter, uplink means communication of the terminal with the base station. The network node may represent the terminal, the base station, or both of them. The configuration may be determined by a network or may mean a rule previously determined in the terminal.

Meanwhile, some of terminals performing the D2D operation are located outside of coverage of the base station, the terminal cannot receive the synchronization provided from the base station. As a result, synchronization between terminals performing the D2D operation is difficult. Since reliability is very important for public safety in the D2D operation, the above synchronization between the terminals may be a problem.

FIG. 15 and FIG. 16 illustrate a situation of transmitting synchronization by a terminal for another terminal according to the present invention.

FIG. 15 illustrates an example of transmitting a synchronization signal by a terminal for another terminal.

Referring to FIG. 15, a user equipment (UE) 2 153 is located in a coverage 154 boundary of the network, and UE 1 152 may be located outside of the coverage 154 of the network.

Since the UE 1 152 is located outside of the coverage 154 of the network, the UE 1 152 cannot directly receive the synchronization signal. In this case, the UE 2 153 may receive the synchronization signal of the network to transfer the synchronization signal to the UE 1 152. That is, UE 2 153 may transfer the synchronization signal of the network to the UE 1 152 by generating and transmitting a synchronization signal aligned with the synchronization signal of the network.

FIG. 16 illustrates another example of transmitting a synchronization signal by the terminal for another terminal.

Referring to FIG. 16, a UE 2 163 will perform a D2D operation with UE 1 162 or UE 3 161. In this case, the UE 2 163 does not receive the synchronization signal or receives the synchronization signal but cannot use the synchronization signal, the UE 2 163 may transmit the synchronization signal by itself.

FIG. 17 illustrates a synchronization signal transmission state reporting method performed by a terminal.

Referring to FIG. 17, a terminal receives configuration information on a synchronization signal transmission state reporting (S210).

The terminal evaluates a reporting event with respect to transmission of the synchronization signal (S220). When the reporting event is satisfied, the terminal reports a transmission state of the synchronization signal.

Hereinafter, the respective steps will be described in detail.

The configuration information on a synchronization signal transmission state reporting (hereinafter referred to as ‘configuration information’) may set the reporting event and information to be included in the configuration information on a synchronization signal transmission state reporting in the terminal.

The terminal may report a synchronization signal state thereof. The terminal may receive configuration what contents are included in the report, when is reported, and how to report according to the configuration information.

According to the configuration, the terminal may report a synchronization signal state thereof if a specific condition is satisfied. The specific event refers to a reporting event for convenience. When configuration of at least one reporting event is received, the terminal evaluates whether an event is satisfied.

For example, the reporting event may be one of following events.

TABLE 2 Event Description Event_S1 Terminal determines to start transmission of a synchroniza- tion signal Event_S2 Terminal determines to previously start transmission of a synchronization signal Event_S3 Terminal determines to stop transmission of a synchronization signal which is currently in progress Event_S4 Terminal determines that transmission of a synchronization signal in progress stops Event_S5 Terminal determines to again start transmission of a synchronization signal which stops Event_S6 Terminal determines that transmission of a synchronization signal restarts

Respective events of the table 2 will be described in detail.

1. Event_S1: A terminal determines an Event_S1 to start transmission of a synchronization signal.

The event allows the terminal to report necessity of transmission of the synchronization signal to a network. If a report triggered according to the event is received, the network may transmit a transmission command to the terminal. The synchronization signal transmission command may include information indicating allowance of transmission of the synchronization signal to the terminal. The configuration information may indicate a radio resource which may be used in transmission/reception of the synchronization signal.

2. Event_S2: The terminal determines that the transmission of the synchronization signal starts.

The event is performed for the purpose of reporting that the terminal starts the transmission of the synchronization signal to the network. If the report triggered according to the event is received, the network may know that the transmission of the synchronization signal starts by the terminal. The network may transmit preset signal to the terminal in order to assist or control the transmission of the synchronization signal to the terminal.

The event may be set when the terminal is allowed to start transmission of the synchronization signal by itself. That is, when the terminal may perform start/stop of transmission of the synchronization signal without an explicit command. The above case excludes to indicate allowing/non-allowing to transmit the synchronization signal by the terminal itself according to the explicit command of a network.

3. Event_S3: the terminal determines to stop transmission of the synchronization signal which is currently in progress.

4. Event_S4: the terminal determines that transmission of a synchronization signal in progress stops.

5. Event_S5: The terminal determines to again start transmission of a synchronization signal which stops

The above events are related to stop/restart of the transmission of the synchronization signal by the terminal.

If there is no new radio resource configuration associated with transmission of the synchronization signal when the terminal receives a command indicating restart of the transmission of the synchronization signal, the terminal may reuse wireless resource configuration which is used in transmission of a previous synchronization signal.

6. Event_S6: the terminal determines that transmission of a synchronization signal restarts

The event may be configured when the terminal is allowed to restart the transmission of the synchronization signal by itself. In this case, it may be allowed to reuse the radio resource configuration which is used to transmit a previous synchronization signal.

Information to be included in a report may be also set in the terminal. For example, the above information may be set by the above configuration information. The report triggered according to a reporting event may include all or a part of state information of transmission of a synchronization signal, an ID of a terminal transmitting the report, an ID list of other terminals to use the synchronization signal, and an indicator to indicate whether there is one of the other terminals to use the synchronization signal for synchronization.

For example, the state information of the transmission of the synchronization signal may be information indicating one of following states.

1) There is a need to start transmission of a synchronization signal, 2) Transmission of the synchronization signal starts, 3) There is a need to start the transmission of the synchronization signal, 4) Transmission of the synchronization signal stops, 5) There is a need to restart the transmission of the synchronization signal, 6) Transmission of the synchronization signal restarts.

The terminal may also receive configuration of information on a reception target of the above report. For example, a terminal transmitting the above report may receive configuration of an ID (or ID list) of a network node which is allowed to receive the report. In this case, the network node may include a different terminal or a base station. When the network node is the base station, the ID may include a cell ID such as a physical cell ID, a layer 2 cell ID, and a global cell ID. When the network node is a terminal, the ID may an ID of the terminal or an equivalent optional ID thereof.

Alternatively, only with respect to a network node to transmit configuration information with respect to the above report, the terminal may transmit the report. That is, if a base station 1 transmits configuration information associated with a report with respect to a state of transmission of the synchronization signal to the terminal, the terminal receiving the configuration information transmits a corresponding report to only the base station 1.

Meanwhile, when a reporting event (or condition), the terminal may be configured how many/often is reported. For example, one of following two methods will be configured.

1) Temporary reporting: if the reporting event (condition) is satisfied, the terminal may be configured to temporarily perform the report.

2) Periodic reporting: if the reporting event (condition) is satisfied, the terminal may be configured to periodically perform the report. A reporting period should be set in the periodic reporting.

Different reporting events may be associated with different reporting configurations, and the reporting configuration may be configured independently from the reporting event configuration. In this case, the relationship between the reporting setting and the reporting event setting is provided.

The reporting configuration may be dynamically provided by a network. Alternatively, the reporting configuration may be previously set. The configuration may include an RAN level configuration. The RAN level configuration means configuration where the base station determines, configures, and provides to a terminal through an RRC message. The configuration may include configuration exceeding the RAN level. For example, the configuration may be performed in an NAS level. In this case, an MME determines, configures, and provides the configuration to the terminal. The configuration may be performed through an OMA DM. In this case, a server of a network may determine and configure the OMA DM to provide the OMA DM to the terminal through an OMA DM process.

The following is a description of a synchronization signal state reporting method with respect to a case of performing start, stop, and restart of transmission of the synchronization signal by the terminal and a case of performing start, stop, and restart of transmission of the synchronization signal only when receiving an explicit command from a network.

FIG. 18 illustrates a synchronization signal transmission state reporting method according to an embodiment of the present invention. It is assumed that UE 1 may perform start, stop, and restart of transmission of the synchronization signal by itself.

Referring to FIG. 18, the UE 1 receives configuration information on the synchronization signal transmission state reporting (S221). It is assumed that for example, Event_S2, Event_S4, and Event_S6 are configured as a reporting event according to the configuration information.

The UE 1 may identify no synchronization signals through measurements (S222).

Alternatively, the UE 1 may detect a synchronization signal (referred to as ‘network synchronization signal’) transmitted by a network. In this case, the UE 1 may determine to transfer the network synchronization signal to another UE neighboring the UE 1. Accordingly, the UE 1 may generate and transmit a synchronization signal (referred to as ‘UE 1 synchronization signal’) thereof aligned with a network synchronization signal. Another UE (e.g., UE 2) obtains a result in the same as in reception of the network synchronization signal according to the UE 1 synchronization signal. Accordingly, the UE 2 may perform synchronization with respect to the network.

Alternatively, the UE 1 may receive the synchronization signal, or cannot detect a synchronization signal which is allowed so that the UE can use. For example, each UE or a UE group may be allowed to use only a specific synchronization signal in a resource pool for the synchronization signal. In this case, although a synchronization signal which is not allowed in the UE is received, the synchronization signal cannot be used.

Although the UE 1 may identify that the UE 2 is close to the UE 1, the synchronization signal to be used by the UE 2 may not be detected.

The UE 1 may identify that another UE (e.g., UE 2) needs synchronization for communicating with another UE (e.g., UE 3) or a base station to start transmission of the synchronization signal (S223). That is, the UE 1 for communication of the UE 2 may determine the transmission of the synchronization signal to transmit the synchronization signal. Other UEs close to the UE 1 as well as the UE 2 may perform synchronization based on the synchronization signal transmitted from the UE 1. Accordingly, the UE 2 may perform a D2D operation with other UEs or communication with a base station.

In this case, since the UE 1 starts the transmission of the synchronization signal, the UE 1 may determine that a result Event_S2 is satisfied. Next, the UE 1 reports that the transmission of the synchronization signal starts to the base station (S224). That is, the base station may transmit state information on the transmission of the synchronization signal indicating that the transmission of the synchronization signal starts to the base station.

After a predetermined time elapses, the UE 1 may detect that the UE 1 is located within coverage of the base station or there is no different UE using a synchronization signal transmitted from the UE 1 (S225).

Next, the UE 1 determines/performs to stop transmission of the synchronization signal (S226). As a result, the Event_S4 is satisfied.

Since a preset reporting event is satisfied, the UE 1 reports stop of the transmission of the synchronization signal to the base station (S227). In other words, the UE 1 transmits state information of the transmission of the synchronization signal indicating the stop of the transmission of the synchronization signal to the base station.

After a predetermined time elapses, the UE 1 may detect no synchronization signals. Alternatively, although the UE 1 receives the synchronization signal, the UE 1 may not detect a synchronization signal which is allowed. For example, the above case is a case where each UE or a UE group may be allowed to use only a specific synchronization signal in a resource pool for the synchronization signal, but may not detect the specific synchronization signal.

Alternatively, the UE 1 may not detect the synchronization signal to be used by another terminal (e.g., UE 2) in a state that another UE is close to the UE 1. In this case, the UE 1 may determine that the UE 2 needs the synchronization signal.

The UE 1 determines/performs restart of transmission of the synchronization signal (S229). In this case, the synchronization signal may be provided for synchronization for a D2D operation between the UE 1 and another UE. Alternatively, the synchronization signal may be provided for synchronization for the D2D operation between other UEs (e.g., UE 2, UE 3). The other UEs may be located outside of coverage of the base station or may be located in coverage of the UE 1. The UE 2 and the UE 3 may perform synchronization based on the synchronization signal provided from the UE 1 and then perform a D2D operation to each other. Alternatively, the synchronization signal may be provided for synchronization for communication between another UE and the base station.

Since the UE 1 restarts transmission of the synchronization signal, the result Event_S6 is satisfied. Since a preset reporting event is satisfied, the UE 1 reports that the transmission of the synchronization signal restarts to the base station (S230). In other words, the UE 1 transmits a report including state information of the transmission of the synchronization signal indicating the transmission of the synchronization signal to the base station.

The above method of FIG. 18 is performed based on a case where the UE 1 may perform transmission/stop/restart of the synchronization signal or the transmission/stop/restart of the synchronization signal is allowed. If the UE 1 cannot perform the transmission/stop/restart of the synchronization signal by itself or the transmission/stop/restart of the synchronization signal is allowed, the UE 1 may be operated according to a following embodiment.

FIG. 19 illustrates a synchronization signal transmission state reporting method according to another embodiment of the present invention. It is assumed that the UE 1 may perform the transmission/stop/restart of the synchronization signal only when explicitly receiving a command from a network.

Referring to FIG. 19, the UE 1 receives configuration information on the synchronization signal transmission state reporting (S231). It is assumed that for example, Event_S2, Event_S4, and Event_S6 are configured as a reporting event according to the configuration information.

The UE 1 may identify no synchronization signals through measurements (S232).

Alternatively, the UE 1 may detect a synchronization signal (refers to ‘network synchronization signal’) transmitted from a network. In this case, the UE 1 may determine to transfer a network synchronization signal to another UE close to the UE 1. Accordingly, the UE 1 may generate and transmit a synchronization signal thereof (refers to ‘UE 1 synchronization signal’) aligned with the network synchronization signal. Another UE (e.g., UE 2) close to the UE 1 obtains the result in the same as in reception of the network synchronization signal according to the UE 1 synchronization signal. Accordingly, the UE 2 may perform synchronization with respect to the network.

Alternatively, the UE 1 receives the synchronization signal transmitted from another UE, but may detect no synchronization signals which are allowed to be used by the UE 1. For example, each UE or the UE group may be allowed to use only a specific synchronization signal in a resource pool for the synchronization signal. In this case, although a synchronization signal which is not allowed in the UE is received, the synchronization signal cannot be used.

The UE 1 may identify that the UE 2 is located close to the UE 1 but a synchronization signal to be used according to the UE 2 may be not detected.

The UE 1 may determine that so that another UE (e.g., UE 2) requires synchronization for communication with another UE (e.g., UE 3) or a base station (S233 S235). In detail, the UE 1 determines to start the transmission of the synchronization signal (A of S233). Accordingly, since the Event_S1 is satisfied, the UE 1 transmits state information on transmission of the synchronization signal indicating that transmission of the synchronization signal starts to the base station (S234). Accordingly, the base station transmits a command to allow start of the transmission of the synchronization signal to the UE 1 (S235). The UE 1 receiving the explicit command starts the transmission of the synchronization signal (D of S233).

That is, in the method of FIG. 18, the UE 1 determines and performs to start transmission of the synchronization signal by itself. However, the difference is that the UE 1 starts an explicit command of a base station and then starts transmission of the synchronization signal in the method of FIG. 19.

Other UEs (e.g., UE 2 and UE 3) close to the UE 1 may perform synchronization using the synchronization signal transmitted from the UE 1 to perform a D2D operation to each other. Alternatively, the UE 2 may perform synchronization using the synchronization signal transmitted from the UE 1 to perform communication with the base station.

After a predetermined time elapses, the UE 1 may detect that the UE 1 is located within coverage of a base station or may detect there is no other UEs to perform synchronization using the synchronization signal transmitted from the UE 1 (S236).

Accordingly, the UE 1 determines and performs stop of transmission of the synchronization signal (S237˜S239). In detail, the UE 1 determines to stop of the transmission of the synchronization signal (A of S237). Accordingly, when a reporting event Event_S3 is satisfied, the UE 1 transmits state information on the transmission of the synchronization signal indicating stop of the transmission of the synchronization signal to the base station (S238). Accordingly, the base station transmits a command to allow stop of the transmission of the synchronization signal to the UE 1 (S239). The UE 1 receiving the explicit command stops the synchronization signal (D of S237).

That is, in the method of FIG. 18, the UE 1 determines and performs to start transmission of the synchronization signal by itself. The difference is that the UE 1 starts an explicit command of a base station and then starts transmission of the synchronization signal in the method of FIG. 19.

After a predetermined time elapses, the UE 1 may detect/identify no synchronization signals (S240). Alternatively, the UE 1 receives the synchronization signal, but may detect no synchronization signals which are allowed to be used by the UE 1. For example, each UE or the UE group may be allowed to use only a specific synchronization signal in a resource pool for the synchronization signal, which is a case of not detecting the specific synchronization signal.

Alternatively, the UE 1 may not detect the synchronization signal to be used by another terminal (e.g., UE 2) in a state that another UE is close to the UE 1. In this case, the UE 1 may determine that the UE 2 needs the synchronization signal.

The UE 1 determines to restart transmission of the synchronization signal and performs restart of the transmission of the synchronization signal (S241˜S243). In detail, the UE 1 determines restart of the transmission of the synchronization signal (A of S241). Accordingly, since the Event_S1 is satisfied, the UE 1 transmits state information on transmission of the synchronization signal indicating that transmission of the synchronization signal restarts to the base station (S242). Accordingly, the base station transmits a command to allow start of the transmission of the synchronization signal to the UE 1 (S243). The UE 1 receiving the explicit command starts the transmission of the synchronization signal (D of S241).

That is, in the method of FIG. 18, the UE 1 determines and performs to start transmission of the synchronization signal by itself. The difference is that the UE 1 starts an explicit command of a base station and then starts transmission of the synchronization signal in the method of FIG. 19.

In this case, the synchronization signal may be provided for synchronization for a D2D operation between the UE 1 and another UE. Alternatively, the synchronization signal may be provided for synchronization for the D2D operation between other UEs (e.g., UE 2, UE 3). The other UEs may be located outside of coverage of the base station or may be located in coverage of the UE 1. The UE 2 and the UE 3 may perform synchronization based on the synchronization signal provided from the UE 1 and then perform a D2D operation to each other. Alternatively, the synchronization signal may be provided for synchronization for communication between another UE and the base station.

FIG. 20 is a block diagram illustrating a wireless device according to an embodiment of the present invention.

Referring to FIG. 20, a terminal 1100 includes a processor 1110, a memory 1120, and an RF unit 1130. The processor 1110 performs the proposed functions, processes and/or methods. For example, the processor 1110 evaluates an event associated with the synchronization signal transmission state. If the event is satisfied, the processor 1110 transmits a report with respect to the synchronization signal transmission state.

The RF unit 1130 is connected to the processor 1110, and sends and receives radio signals.

The processor may include Application-Specific Integrated Circuits (ASICs), other chipsets, logic circuits, and/or data processors. The memory may include Read-Only Memory (ROM), Random Access Memory (RAM), flash memory, memory cards, storage media and/or other storage devices. The RF unit may include a baseband circuit for processing a radio signal. When the above-described embodiment is implemented in software, the above-described scheme may be implemented using a module (process or function) which performs the above function. The module may be stored in the memory and executed by the processor. The memory may be disposed to the processor internally or externally and connected to the processor using a variety of well-known means. 

What is claimed is:
 1. A synchronization signal transmission state reporting method performed by a user equipment (UE) in a wireless communication system, the method comprising: evaluating an event related to the synchronization signal transmission state by the UE; and when the event is satisfied, transmitting a report for the synchronization signal transmission state.
 2. The method of claim 1, wherein the report is transmitted to a base station or another UE.
 3. The method of claim 1, wherein the event is at least one of events included in a following table: Event Description Event_S1 UE determines to start transmission of a synchronization signal Event_S2 UE determines to previously start transmission of a synchronization signal Event_S3 UE determines to stop transmission of a synchronization signal which is currently in progress Event_S4 UE determines that transmission of a synchronization signal in progress stops Event_S5 UE determines to again start transmission of a synchronization signal which stops Event_S6 UE determines that transmission of a synchronization signal restarts


4. The method of claim 1, wherein the report comprises at least one of state information indicating the synchronization signal transmission state of the UE, an identity (ID) of the UE, and an ID of a reception subject of the report.
 5. The method of claim 1, wherein configuration information on a synchronization signal transmission state reporting is received, and information to be included in the event and the report is configured in the UE according to the configuration information.
 6. The method of claim 1, wherein the UE determines and performs start, stop, and restart of the synchronization signal.
 7. The method of claim 6, wherein the synchronization signal is used in synchronization for a D2D operation with another UE close to the UE, synchronization for the D2D operation between other UEs close to the UE, or synchronization for communication of another UE close to the UE with a network.
 8. The method of claim 1, wherein the UE performs start, stop, and restart of transmission of a synchronization signal by receiving an explicit command from a network.
 9. The method of claim 8, further comprising: when the event is satisfied, transmitting the report to the network; and receiving a command related to the transmission of a synchronization signal as a response to the report from the network; and starting, stopping, and restarting the transmission of a synchronization signal according to the command.
 10. The method of claim 8, wherein the synchronization signal is used in synchronization for a D2D operation with another UE close to the UE, synchronization for the D2D operation between other UEs close to the UE, or synchronization for communication of another UE close to the UE with a network.
 11. A user equipment (UE) comprising: an RF unit configured to send and receive radio signals; and a processor connected to the RF unit to be operated, wherein the processor evaluates an event related to the synchronization signal transmission state by the UE; and transmits a report for the synchronization signal transmission state when the event is satisfied. 